Question: Mark can make $42$ birthday cakes in $7$ days. How many birthday cakes can Mark make in $5$ days?
Solution: We're trying to find the number of cakes Mark can make in $5$ days. If in ${7}$ days, Mark can make ${42}$ cakes, then in $1$ day, he can make $6$ cakes.  So, in ${5}$ days, Mark can make ${30}$ cakes.